This invention relates to the recovery of metal from solution, and in particular to an electrolytic cell for the recovery of metal from metal rich solution contained therein by plating onto a cathode thereof. The invention finds particular, though not exclusive, application in the electrical and mechanical connection of cathodes for use in an electrolysis cell used in the recovery of silver from photo-processing solutions.
For convenience and by way of example only, the invention will be described with respect to the recovery of silver from photo-processing solutions, it being understood that it has more general application.
In electrolytic silver recovery apparatus, current is passed between the anode and cathode of a cell, and silver from the solution contained therein, for example a photographic fixing solution, becomes plated onto the cathode. The amount of silver on the cathode increases with usage, and ultimately the maximum capacity of silver plating is reached. At this stage it is necessary to remove the cathode from the cell and either to replace it with a fresh one, or alternatively to detach the silver and to replace the original cathode. Whilst there are cost savings involved in reusing the same cathode, substantial labor costs can be incurred in removing and cleaning the cathode. To minimize the inconvenience of having to separate the silver from the cathode, it is known to use a disposable cathode which is smelted together with the silver thereon in a subsequent refining process. Examples of this are disclosed in U.S. Pat. Nos. 4,440,616, 5,203,979, and 5,370,781. In this way, although the labor cost associated with separating the silver from the cathode is reduced, the user is still required to undertake the messy and inconvenient operation of having to remove from the cell a cathode that is not only coated with silver but also is wet with the solution.
U.S. Pat. No. 4,834,849 discloses an apparatus for recovering a conductive metal from a liquid that contains that metal, in which the container is constructed of a plastics material and the cathode is provided in the form of a thin film applied to the inner surface thereof. With this construction, the entire container can be placed in a smelting furnace on completion of the de-metallizing operation. Whilst this avoids the need to take the fully-loaded cathode out of the cell, the user is required to drain the solution from the container and subsequently to refill the replacement container.
U.S. Pat. No. 4,372,829 discloses an apparatus for removing metal from solution, in which the cathode of the arrangement is mechanically attached to the lid of the cell, and is thus removable therewith. In this way, the user is not required to make contact with the liquid nor to drain or to refill the cell. Since the cathode is connected only to the lid, electrical connection is made therethrough. This is achieved by a plurality of studs that are welded to the top edge of the cylindrical cathode, extend through apertures in the lid and are secured by external nuts. However, this is an expensive construction and does not lend itself to use with a disposable cathode.
U.S. Pat. No. 4,280,884 discloses another approach to connecting the cathode to the lid of the cell. The stainless steel cylindrical cathode is of sufficient wall thickness for it to be retained by bolts extending axially thereinto through the lid. However, such a construction does not lend itself to the production of a low cost cell, for example, one using non-metallic cathode materials.
U.S. Pat. No. 5,370,781, referred to above, provides a solution to the problem of connection to a thin flexible cathode, in the form of a sheet that is conductive on at least one surface, by depending on shaping of the lid to press the cathode outwards onto a metal contact located in the side wall of the cell. However, since the cathode is not mechanically connected to the lid, the user has to pull the cathode, which will be wet and fully loaded with silver, out of the cell by hand or by means of a tool.
Problem to be Solved by the Invention
It is one object of the present invention to provide an electrolytic cell in which electrical connection is made to a fully-loaded cathode, preferably a disposable cathode, in such a way that the cathode may be removed from solution in a cell with the minimum of inconvenience to the user. In particular, it is an object of the invention that any contact of the user with the metal on the cathode of the cell or with any solution remaining thereon, is minimized, and preferably is obviated.
In accordance with one aspect of the present invention, there is provided an electrolytic cell for the recovery of metal from solution contained therein by plating onto a cathode thereof, the cell comprising:
an open-topped container for containing the solution;
a closure arrangement for closing the top of the container to seal the solution therein; and
a cathode; wherein
the closure arrangement comprises (a) electrically insulating support means for supporting the cathode such that it extends into the solution in the container, for example around the inside thereof, and (b) electrical connection means that comprises a first part, which may be contained within the closure arrangement, in electrical contact with the cathode, and a second part that extends from the first part and out through the closure arrangement.
The cathode may extend around the support means such that the inner surface of the cathode engages an exposed outer surface of the first part of the electrical connection means.
Preferably the first part of the electrical connection means is substantially annular and is embedded in the support member.
The support means may be integral with the closure arrangement, or may be separate from, and mounted on, the closure arrangement. The closure arrangement may comprise a lid of the container that is secured thereto, for example by being screwed thereon or attached by means of a bayonet fitting or a clamp, so as to seal the container against escape of the solution.
In another embodiment, the support member is urged down onto an internal recess or step of the container by the closure arrangement, for example by a lid thereof, which may be hinged to the container.
The cathode may be compressed onto at least a portion of the support means, thereby making electrical contact between an inner surface of the cathode and the first part of the electrical connection means, preferably by an annular compression ring.
In another embodiment, an inner portion of the support means may urge the cathode outwards into gripping contact with an outer portion thereof. The electrical connection means may be mounted on the outer portion of the support means.
In a preferred embodiment, the cathode is formed into an open-right cylinder having a conductive inner surface and makes electrical contact with an annular connection ring that forms the first part of the electrical connection means. The ring may be mounted on a cylindrical boss that depends downwardly from the lid.
In another embodiment, one of the cathode and support member comprises at least one projection that is arranged to engage with at least one aperture of the other of the cathode and support member, whereby the cathode may be mounted around the support member under tension.
The cell may be provided with an air bleed hole to facilitate introduction of the cathode into the liquid-filled container and to minimize the amount of liquid sucked out on its removal therefrom. The bleed hole extends from within the cathode space, and may terminate within or outside the container.
The cathode may be retained on the support means by a cam arrangement.
The cathode, preferably in cylindrical form, may be of a thin and flexible construction, and is advantageously formed from a conductive material supported by, for example laminated with, a non-conductive substrate. Preferably, the cathode comprises graphite supported by, for example in the form of a foil laminated to, a polyester backing sheet. With this construction, a low cost, easily disposable cathode can be achieved.
Advantageous Effects of the Invention
It will be appreciated that with both mechanical and electrical connection to the cathode being made upwardly to the closure arrangement, the cathode may be conveniently and safely removed from the container by the user, minimizing exposure to the plated metal and to the solution.
The cathode can conveniently be formed from a low cost flexible sheet of material, and when silver, non-metallic, or low in metal content (other than silver), may be refined together with the metal plated thereon.
When the cathode is formed of a non-metallic material, electrical connection thereto is still advantageously made by metal components. By arranging that the solution does not contact such components, for example by embedding them, plating thereon by metal from the solution is avoided.